Radiographic Exposure
• Exposure Factors influence and determine
the quantity and quality of the x-radiation
to which the patient is exposed.
• Radiation quantity refers to the radiation
intensity referred to as mR or mR/ mAs.
• Radiation Quality refers to the beam
penetrability and measured in HVL.
Radiographic Exposure
• The radiographic exposure factors are
under the control of the operator except
for those fixed by the design of the x-ray
machine.
• There are two choices for focal spot.
• With the exception of compensating filters,
added filtration is fixed.
• The type of high voltage power is also
fixed.
Exposure Factors Controlled by
the Operator
• kVp
• mA times Exposure Time = mAs
• Determines the quality and quantity of the
exposure
• SID, Focal Spot and Filtration are
secondary factors
kVp
• As we have discussed in the laboratory,
kVp controls radiographic contrast.
• kVp determines the ability for the beam to
penetrate the tissue.
• kVp has more effect than any other factor
on image receptor exposure because it
affects beam quality.
kVp
• To a lesser extent it also influences the
beam quantity.
• As we increase kVp, more of the beam
penetrates the tissue with higher energy
so they interact more by the Compton
effect.
• This produces more scatter radiation
which increases image noise and reduces
contrast.
kVp
• 50 kV 79% is photoelectric, 21% Compton, < 1%
no interaction
• 80 kVp 46% is photoelectric, 52% Compton 2%
no interaction
• 110 kVp 23% photoelectric, 70% Compton, 7%
no interaction
• As no interaction increases, less exposure is
needed to produce the image so patient
exposure is decreased.
mA
• 1 Ampere = 1 C/s = 6.3 x 1018 electrons/ second.
• The mA selected for the exposure determines
the number of x-rays produced.
• The number of x-rays are directly proportional to
the mA assuming a fixed exposure time.
• 100 mA produced half the x-ray that 200 mA
would produce.
mA
• Patient dose is also directly proportional to
the mA with a fixed exposure time.
• A change in mA does not affect kinetic
energy of the electrons therefore only the
quantity is changed.
mA
• Many x-ray machines are identified by the
maximum mA or mAs available.
• A MP 500 has a maximum mAs of 500
mAs.
• A Universal 325 has a maximum mA of
300 and maximum kVp of 125
mA
• More expensive three phase machines will
have a higher maximum mA.
• A General Electric MST 1050 would have
1000 mA and 150 kVp.
Exposure Time
• The exposure time is generally always
kept as short as possible.
• This is not to reduce patient exposure but
to minimize motion blur resulting from
patient movement.
• This is a much greater problem with
weight bearing radiography.
Exposure Time
• Older machine express time as a fraction.
• Newer machines express exposure time
as milliseconds (ms)
• It is easy to identify the type of high
voltage generation by looking at the
shortest exposure time.
Exposure Time
• Single phase half wave rectified fasted
exposure time is 1/60 second 17 ms.
• Single phase full wave rectified fastest
exposure time is 1/120 second or 8 ms
• Three phase and high frequency can
provide exposure time down to 1 ms.
mAs
• mA and exposure time is usually
combined and used as one factor
expressed as mAs.
• mAs controls radiation quantity, optical
density and patient dose.
• mAs determine the number of x-rays in the
beam and therefore radiation quantity.
• mAs does not influence radiation quality.
mAs
• Any combination of mA and time that will
give the same mAs should provide the
same optical density on the film. This is
referred to as the reciprocity law.
• As noted earlier for screen film
radiography, 1 ms exposure and exposure
longer than 1 seconds do not follow this
rule.
mAs
• On many modern machines, only mAs can
be selected. The machine automatically
gives the operator the highest mA and
shortest exposure time.
• The operator may be able to select mA by
what is referred to as Power level.
mAs
• mAs is one way to measure electrostatic
charge. It determines the total number of
electrons.
• Only the quantity of the photons are
affected by changes in the mAs.
• Patient dose is therefore a function of
mAs.
mAs
• If we know the mR/mAs, multiply that
figure times the mAs. or
• If we know the mR for a given exposure
at a given kVp, we can divide the
exposure by the mAs to get the mR/ mAs.
• To compute exposure we need to know
what the mR/mAs is for the kVp used and
the SID.
Distance
• Distance affects the exposure of the
image receptor according to the inverse
square law.
• Distance affects the intensity of the x-ray
beam at the film but has no effect on
radiation quality.
Inverse Square Law
– mAs (second exposure)
SID2 2nd
exposure
– ----------------------------= ------------------------– mAs (first exposure)
SID2 1st exposure
Distance
• The most common source to image
distances are 40” (100 cm) and 72”(182
cm)
• Since SID does not impact the quality of
the beam, adjustments to the technical
factors are made with the mAs.
• To go from 40” to 72” increase the mAs
3.5 time.
Distance
• Increasing the distance will impact the
geometric properties of the beam.
• Increased SID reduces magnification
distortion and focal spot blur.
• With the need to increase the mAs 3.5
times for the 72” SID, tube loading
becomes a concern.
Distance
• 72” SID is used for Chest radiography and
the lateral cervical spine to reduce
magnification.
• 72” SID used for the full spine to get a 36”
beam.
Imaging System Characteristics
• Operator has limited control.
• The following will impact the technical
factors based upon the type of machine.
– Focal Spot Size
– Filtration
– High-voltage Generation
Focal Spot Size
• Most machines limited to two focal spot
sizes.
• Common office focal spots are 1.0 mm for
the small and 2.0 mm for large.
• Highly detailed radiography such as
mammography use micro-focus tubes with
0.1 mm and 0.3 mm focal spot sizes.
Focal Spot Size
• The focal spot size limits the tube’s
capacity to produce x-rays. The electrons
and resulting heat are placed on a smaller
portion of the x-ray tube.
• The mA is therefore limited for the small
focal spot. This results in longer exposure
times with greater chance of patient
movement.
Focal Spot Size
• For single phase machines, the small focal
spot use is limited to extremities and the
cervical spine.
• With high frequency, most views can be
done on the small focal spot except for
larger patient and ones that cannot hold
still.
• My limit is exposure times less than 1/2 s.
Focal Spot Size
• If the mA is properly calibrated, the focal
spot will have no impact on the quantity or
quality of the beam.
Filtration
• All x-ray beams are affected by the
filtration of the tube. The tube housing
provides about 0.5 mm of filtration.
• Additional filtration is added in the
collimator to meet the 2.5 mm of aluminum
minimum filtration required by law.
• 2.5 mm is required for 70 kVp.
Filtration
• 3.0 mm is required for at 100 kVp.
• 3.2 mm is required for operations at 120
kVp.
• Most machines now are capable of over
100 kVp operation.
• We have no control on these filters.
Filtration
• Chiropractic radiography is a leader in the
use of compensating filters. We have total
control over compensating filtration.
• In areas of the body with high subject
contrast or wide differences in density,
compensating films improve image quality
and reduce patient exposure.
High-voltage Generation
• You will determine the type of high-voltage
generation when you purchase your x-ray
machine.
• The type of generator will determine the
efficiency of the generator or the amount
of ripple in the wave form.
• Single phase has 100% ripple.
Three Phase Generation
• Three phase has a 14% so it is significant
improvement in efficiency increasing both
quality and quantity of the beam.
• More x-rays per mAs with higher energy.
• Cost to provide 3 phase power is very high
so not practical in office.
High Frequency Generation
• Virtually no ripple ( less than 1%.)
• Inexpensive and can use normal incoming
power.
• Provides significant reduction is mAs or
kVp compared to single phase. Reduction
of mAs by 50% compared to single phase
techniques.
Chapter 19 Radiographic
Quality
• Radiographic Quality refers to the fidelity
with which the anatomic structures being
examined are images on the film.
• Three main factors:
– Film Factors
– Geometric Factors
– Subject Factors
Radiographic Quality
• Characteristic of radiographic quality:
– Spatial Resolution (Recorded Detail)
– Contrast Resolution (Visibility of Detail)
– Noise (Visibility of Detail)
– Artifacts
Spatial Resolution
• Spatial Resolution is the ability to image
small structures that have high subject
contrast such as bone-soft tissue
interface.
• When all of the factors are correct,
conventional radiography has excellent
spatial resolution.
Contrast Resolution
• Contrast resolution is the ability to
distinguish structures with similar subject
contrast such as liver-spleen, fat-muscle.
• Computed tomography and MRI have
excellent contrast resolution. Convention
radiology is fair to poor.
Noise
• Noise is an undesirable fluctuation in
optical density of the image. Two major
types:
– Film Graininess- no control over
– Quantum Mottle- some control over
Film Graininess
• Film graininess refers to the distribution in
size and space of the silver halide grains
in the film emulsion.
• Similar to photographic film. 400 ASA film
is more graininess than 100 ASA film.
• Similar to structure mottle that refers to
the size and shape of the phosphors in the
intensifying screens.
Quantum Mottle
• Quantum mottle refers to the random
nature of how the x-rays interact with the
image receptor.
• It is the primary form of radiographic
noise.
• The use of high mAs and low kVp reduced
quantum mottle.
Quantum Mottle
• Very fast screens have higher quantum
mottle because it takes fewer x-rays to
make the image.
Speed
• Resolution and noise are intimately
connected with speed.
• While the speed of the images receptor is
not apparent on the image, it influences
both resolution and noise.
Radiographic Quality Rules
• Fast Image receptors have high noise and
low spatial and contrast resolution.
• High spatial and contrast resolution
require low noise and slow image
receptors.
• Low noise accompanies slow image
receptors with high spatial and contrast
resolution.
Film Factors of Quality
• Characteristic curve
– Density
– Contrast
– Latitude
• Processing
– Time
– Temperature
Sensitometry
• Sensitometry is the study of the
relationship between the intensity of
exposure of the film and the blackness
after the film is processed.
• Unexposed film is clear with a blue tint
after processing.
• Exposed film is black after processing.
Sensitometry
• Two principles involved.
– Exposure of the film
– Amount of light transmitted through the
processed film of optical density.
• Used to describe the relationship of
radiation exposure and blackness or
density on the film.
Characteristic Curve
• This relationship is
called the
characteristic curve or
H & D curve of the
film.
• H & D stands for
Hurter and Driffield.
Parts of the Characteristic
Curve
• Toe and shoulder
where large changes
in exposure results in
small changes in OD.
• Very high and very
low variations of
exposure make very
small changes in
density.
Parts of the Characteristic
Curve
• The straight line or
intermediate area is
where very small
changes in exposure
results in large
changes in density.
• This is the important
part of the curve in
radiography.
Log Relative Exposure (LRE)
• X-ray films responds
to a wide range of
exposure from 1 mR
to 1000 mR.
• Exposure is
represented on
logarithmic manner.
Optical Density Range
• The optical density
range is from 0.0 for
no density to 4.0 for
absolute black.
• Useful range in
general radiography
is from 0.5 to 2.25.
• Image range is 0.5 to
1.25 OD
Base fog or base density
• The tint of the base of
the film and the
inadvertent exposure
of the during
processing.
• Range is from 0.1 to
0.3. Should be never
above 0.30 most is .
21 OD
Items that Impact Base Fog
• Film storage
• Film exposure to wrong spectrum of light
or light intensity.
• Chemical contamination.
• Improper processing.
• High Base fog levels reduce contrast.
Contrast
• Radiographic Contrast is the combined
result of image receptor contrast and
subject contrast.
• Image receptor contrast refers to the
contrast inherent in the film and influenced
by the processing of the film.
Contrast
• Subject contrast is determined by the size,
shape and x-ray attenuating
characteristics of the subject being
examined and the energy (kVp) of the xray beam.
Image Receptor Contrast
• Inherent to the film and screen
combination but is influenced by:
– Range of Optical Density
– Film Processing Technique
• Film type is determined by the type of
intensifying screens used but many
dealers sell off brands of film.
Image Receptor Contrast
• The slope of the
straight line portion of
the H & D curve is the
receptor contrast.
• The average gradient
is a straight line
drawn between the
densities of 0.25 and
2.00 + base fog.
Average Gradient
• The average gradient
is a straight line
drawn between 0.25
OD and 2.0 OD
above base plus fog.
• This is the normal
range of density in a
radiograph
Speed
• Speed is the ability of
the receptor to
respond to low x-ray
exposure.
• The H & D curse is
useful in comparing
speed when selecting
film or screens.
Speed
• A relative number of 100 given to Par
Speed Calcium Tungstate Screens.
• High Speed Calcium Tungstate has a
speed of 200. Half of the exposure is
needed to produce the same image.
• Rare earth screen film combinations range
is speed from 80 to 1600.
Speed
• By knowing the Speed, sometimes
referred to as the Relative Speed Value, it
is easy to convert the technical factors for
one speed to another speed.
LATITUDE
• Latitude can be
observed on the H &
D curve.
• Latitude refers to the
range of exposure
that will produce a
diagnostic range OD.
Latitude
• Latitude and Contrast
are inversely
proportional.
• Wide latitude has a
wide gray scale or low
contrast. (B)
• Narrow latitude has a
short scale or high
contrast. (A)
Latitude
• Latitude is designed into some screen and
film combinations. With wide latitude, the
error factor in technique is wider.
• Latitude can also be impacted by the
technical factors.
Film Processing
• Radiographic Quality
is impacted by film
processing
parameters.
• The developer must
be at the proper
concentration and at
the correct
temperature.
Film Processing
• The film must also
spend the correct
amount of time in the
developer.
• This is the time &
temperature
relationship.
Processing
• Speed and base fog increase with the
temperature.
• Contrast will increase to a point and then
drop with the base fog increase.
• Manufactures set processing parameters
to optimize speed, contrast and low base
fog.
Processing
• In 9th Quarter we will discuss processor
quality control in detail.
End of Lecture
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